B3GAT3-related linkeropathy and an in-frame homozygous deletion in an adult patient

Proteoglycans (PGs) are complex macromolecules consisting of a core protein and glycosaminoglycan (GAG) side chains. PGs are important for the constitution and functioning of the connective tissue. The normal composition of the GAG side chains defines the nature of the PGs and a wide range of biological events. Deficiencies of specific enzymes involved in the linkage of GAGs to the core protein to form functional PGs, lead to a heterogeneous disease group called Linkeropathies. This is a group of multisystem conditions characterized by different phenotypes that include skeletal dysplasia and various extra-skeletal features: developmental delay/intellectual disability, ophthalmological abnormalities including blue sclerae, facial characteristics, cardiac defects, abdominal wall defects (hernias), cutis laxa, hypermobility and hypotonia. The conditions show variable severity and often overlapping phenotypes. The enzyme β-1,3-glucuronyltransferase 3, encoded by B3GAT3, is involved in the linkage process to form functional PGs. Biallelic pathogenic variants in B3GAT3 hence lead to Linkeropathy due to loss of function or decreased activity of this enzyme. We describe a 22-year-old female patient, born of consanguineous parents. The disease history includes congenital severe joint malalignment of elbows, hips, knees and feet, hypermobility, severe kyphoscoliosis, osteoporosis with multiple fractures in childhood, congenital diaphragmatic hernia, minor dental anomalies, digital malformations, and characteristic facial features. Whole exome sequencing was performed, and homozygosity for a novel in-frame deletion in B3GAT3, (c.61_63delCTC (p.(Leu21del))) was detected. Both unaffected parents (double second cousins) were shown to be heterozygous carriers. This is the first report to describe homozygosity for this specific in-frame deletion in B3GAT3 (p.(Leu21del)). We present a young adult phenotype and a summary of previous reported patients with other biallelic B3GAT3-variants for comparison. Previously described patients of B3GAT3-deficiency were, however, all children with phenotypes ranging from prenatal manifestation and early lethality to less severe. We suggest that this novel homozygous in-frame deletion in B3GAT3 may be the cause of a recessive form of Linkeropathy.